(1) Field of the Invention
The present invention relates to a metal vapor discharge lamp, specifically to a metal vapor discharge lamp, and a lighting apparatus having the metal vapor discharge lamp.
(2) Description of the Related Art
The arc tube contained in the metal halide lamp includes a transparent container in which a halogenated metal is sealed as a light emission metal. The transparent container contains a pair of electrodes that are deposited to face each other. The metal halide lamp emits light at a high temperature when the electrodes receives power supply from outside and discharge electricity.
Conventionally, many arc tubes have been made of quarts glass. In recent years, however, arc tubes made of alumina ceramic are often used since alumina ceramic is superior to quarts glass in heat resistance.
To seal the electrodes in an arc tube made of quarts glass, heat and pressure are applied to both ends of the arc tube so that the ends are crushed. In the case of the arc tubes made of alumina ceramic, a container, which is divided into a main tube portion and narrow tube portions extending out from both ends, is first prepared. Two power transmission members are then respectively inserted into the container through the narrow tube portions. A sealing material such as a frit glass in a molten form is then poured into spaces between the inner surfaces of the narrow tube portions and the power transmission members at both ends, so that the arc tube is sealed by the sealing material (Japanese Laid-Open Patent Application No. S57-78763).
Meanwhile, alumina ceramic arc tubes have various advantages, and thus are expected to achieve high-performance lamps.
For example, since alumina ceramic arc tubes can emit light at a higher temperature than quarts glass arc tubes, it is possible to increase the vapor pressure of a material that is to be enclosed in the arc tubes. This is advantageous to achievement of both color rendering and high efficiency.
Also, alumina ceramic has higher reactivity with the halogenated metal enclosed in the arc tube than does quarts glass. This is advantageous to extension of life of the metal halide lamp.
However, metal halide lamps using such an alumina ceramic arc tube have a problem that the color temperature changes during the lamp life. That is to say, even though the metal halide lamps have enough color temperature characteristics to maintain predetermined color temperatures for the lamps at the beginning, the more the lamps are lighted, for example, for 100 hours, 1,000 hours and so on, the lower the color temperature characteristics are.
The reason for this is considered as follows.
In alumina ceramic arc tubes, which are sealed in a manner described above, each main tube portion side of the space between each narrow tube portion and each power transmission member is not filled with the sealing material.
The light emission metal in liquid form gradually slips into the spaces while the lamps are lighted. Especially, when such a lamp is lighted with electrodes being held vertically, the light emission metal enclosed in the arc tube sinks into the space that is lower than the main tube portion of the container.
As the metal sinks into the space, the amount of the metal that contributes to the lighting in the discharge space is reduced. This prevents enough vapor pressure of the metal from being provided, resulting in a change of color temperature.
One might think that this problem could be solved by enclosing enough amount of the light emission metal to prevent the color temperature change. However, when the light emission metal is enclosed too much, the reaction among the metal, electrodes, alumina, and sealing material is promoted. This decreases the lamp life.
Alternatively, the sealing material may be poured into the spaces deeper to reduce the amount of the light emission metal sinking into the spaces. In this case, however, ends of the inserted sealing material come close to the discharge space where the temperature rises to a considerable extent. This promotes the reaction between the sealing material and the light emission metal, resulting in decreased lamp life. Furthermore, cracks are apt to occur to the sealing material inserted in the spaces.